Does it fit into the double's box and extend beyond it at one end (or
both)?

I strongly suspect that any bits of the faceplate that protrude wider than
the double back box are no deeper than the depth on the faceplate itself.
Looking at the picture, there's a good few cm either end that could
overhang, especially the switch, which looks like it wouldn't need to stick
out the back.

Does it stick out from the wall much more than a standard socket?

It does look a little deeper on the picture, presumably to squeeze the
switch in.

Just guessing, though.

Christian.